Method for improving paint adhesion on stainless steel

ABSTRACT

This invention relates to a method for treating the surface of stainless steel to improve the adhesion of paints and lacquers applied thereto. An aqueous solution of trisodium phosphate is applied to the stainless steel followed by rinsing and thereafter applying a hot aqueous solution of sodium dichromate and calcium fluoride and again rinsing.

nited States Patent Inventor William G. Renshaw, deceased late of 1508 Second St., Natrona Heights, Pa. 15065 by Bernice M. Renshaw, exeeutrix Appl. No. 759,820

Filed Sept. 9, 1968 Patented Sept. 21, 1971 METHOD FOR IMPROVING PAINT ADHESION ON STAINLESS STEEL 6 Claims, We Drawings US. Cl! 148/6.16, l48/6.2 Int. Cl C231 7/26 Field of Search 148/616, 6.2

[56] References Cited UNITED STATES PATENTS 2,322,205 6/1943 DeLong 148/62 X 2,613,165 10/1952 Fischer 148/62 2,858,245 10/1958 Carroll et al 148/62 3,112,231 11/1963 Schuster et a1. 148/62 Primary Examiner-Ralph S. Kendall Attorneys -Bichard Speer and Vineent G. Giola METHOD FOR IMPROVING PAINT ADHESION N STAINLESS STEEL For some applications, it is desirable to apply a coating of paint, lacquer or the like to the surface of stainless steel, however, paint does not normally adhere very well to stainless steel. As a result of the poor adherence, the painted surface will chip and peel relatively easily.

The present invention provides a method of treating stainless steel to improve the adherence of paint to the stainless steel surface. This is accomplished in accordance with the invention by applying an aqueous solution of trisodium phosphate to the stainless steel surface, rinsing and thereafter applying a hot solution of sodium dichromate and calcium fluoride and again rinsing. Although it is not known how the above described treatment functions to improve the adherence of the paint, extensive testing has nevertheless demonstrated the usefulness of this process for this purpose.

It is desirable to employ a trisodium phosphate solution heated to a temperature above about 150 F., preferably about 180 F. After application of the solution, the stainless steel surface is rinsed in water. Following the trisodium phosphate treatment, the steel is immersed in a hot, preferably, boiling solution of sodium dichromate and calcium fluoride after which the steel is again rinsed.

The following examples are provided to illustrate the practice of the invention:

Samples of type 430 and type 201 stainless steel sheet cut to 4 inches X 6 inches were prepared and given three different treatments for comparison. One group of samples was simply degreased in acetone; a second group of samples was given a trisodium phosphate treatment with a percent solution at 180 F. for ID minutes followed by hot and cold water rinses; a third group of samples was treated in accordance with a preferred embodiment of the invention by subjecting them to a trisodium phosphate treatment as described above followed by immersion in a mixed solution of sodium dichromate and calcium fluoride. The dichromate-fluoride solution was prepared by mixing 100 grams of sodium dichromate and 3.8 grams of calcium fluoride in 1,253 cc. of water. After the dichromate-fluoride treatment the samples were given a final rinse. The treated samples, after drying, were painted using a paint composition known as AL-570. The painted samples were dried to evaporate the solvent following which they were baked for 30 minutes at 300 F. After baking, the samples were allowed to stand for 24 hours to effect aging of the paint. To test the adherence of the paint, the treated samples were subjected to a standard water soak test involving immersion into distilled water at 95 F. for varying times.

The adhesive quality of the samples treated as described above was checked by applying a cellophane adhesive tape to the painted surface after 2 hours immersion in the water soak test and again following 24 hours of immersion in the water soak test. The samples which had been treated with acetone were stripped of paint by removal of the adhesive tape after only 2 hours exposure to the water soak test. Substantially similar results were obtained with the samples which had been given the trisodium phosphate treatment alone, i.e. not followed by the sodium dichromate-calcium fluoride treatment.

In contrast to the above the samples which were first degreased in trisodium phosphate and then treated in the sodium dichromate-calcium fluoride solution were found to possess a marked improvement in the adhesive quality of paint applied to the stainless steel surface. Removal of the tape after 2 and 24 hours of water soak test did not result in peeling or chipping of the paint on the stainless steel surface.

As further evidence of the improvement in paint adhesion made possible by the present invention, the treated samples were scratched to break the paint surface and examined. In a test designed to evaluate painted samples which are scratched, they are given a water soak for hours and inscribed immediately to expose base metal. Thereafter, the scratched samples are subjected to an air blast of 25 pounds air pressure with 3/16 inches orifice at an angle of 45 to the surface to try to peel the paint off.

he samples given the trisodium phosphate and sodium dichromate-calcium fluoride treatments demonstrated remarkable adhesion of the paint to the stainless steel surface. Those samples which were treated by merely degreasing in acetone or by the trisodium phosphate solution without the follow up dichromate-fluoride solution treatment did not possess adequate paint adhesion and the paint chipped and peeled from the surface.

It is apparent from the above that various changes and modifications may be made without departing from the invention. Accordingly, the scope of the invention should be limited only by the appended claims.

It is claimed:

1. A method of treating the surface of stainless steel to improve the adhesion of paint, lacquer, and the like thereto which comprises applying to the stainless steel surface an aqueous solution of trisodium phosphate, rinsing to remove the trisodium phosphate solution, applying a hot aqueous solution of sodium dichromate and calcium fluoride and thereafter rinsing to remove the dichromate and fluoride solution.

2. A method according to claim 1 wherein said trisodium phosphate solution is at a temperature above about F.

3. A method according to claim 2 wherein said solution is a 5 percent trisodium phosphate solution.

4. A method according to claim 2. wherein a 5 percent trisodium phosphate solution at a temperature of l80 F. is applied to the stainless steel surface for about 10 minutes.

5. A method according to claim I wherein said sodium dichromate-calcium fluoride solution is a boiling solution.

6. A method according to claim 5 wherein said sodium dichromate-calcium fluoride solution is applied by immersing the stainless steel therein. 

2. A method according to claim 1 wherein said trisodium phosphate solution is at a temperature above about 150* F.
 3. A method according to claim 2 wherein said solution is a 5 percent trisodium phosphate solution.
 4. A method according to claim 2 wherein a 5 percent trisodium phosphate solution at a temperature of 180* F. is applied to the stainless steel surface for about 10 minutes.
 5. A method according to claim 1 wherein said sodium dichromate-calcium fluoride solution is a boiling solution.
 6. A method according to claim 5 wherein said sodium dichromate-calcium fluoride solution is applied by immersing the stainless steel therein. 